A nutrient balance trial was conducted to examine the effects of adding phytase (500 FTU/kg) and 20% DDGS to corn-soybean meal based diets on DM, N, and P digestibility and excretion in grow-finish pigs. Thirty two grow-finish pigs (79.3 ± 1.63 kg) were utilized in 2 x 2 factorial arrangement of treatments. Four experimental diets consisted of corn-soybean meal (CS) without phytase (-PHY), CS with phytase (+PHY), CS+DDGS-PHY, and CS+DDGS+PHY. Pigs were fed their respective experimental diets for a 10d adaptation period followed by a 5d total collection period. There was no difference (P = 0.17) in DM digestibility of pigs fed CS-PHY (88.5%), CS+PHY (88.9%), DDGS-PHY (87.3%), and DDGS+PHY (86.7%), resulting in similar (P = 0.16) DM excretion (g/d) in pigs fed CS-PHY (240), CS+PHY (234), DDGS-PHY (269) or DDGS+PHY (278). Nitrogen digestibility in pigs fed CS-PHY (85.2%), CS+PHY (87.6%), DDGS-PHY (85.4%), and DDGS+PHY (85.8%) was not affected (P = 0.40), but there was a trend (P = 0.09) in for reduced fecal N excretion (g/d) in pigs fed diets containing phytase. Urinary N excretion (%) was similar (P = 0.16) in pigs fed CS-PHY (0.89), CS+PHY (0.48), DDGS-PHY (0.61) and DDGS+PHY (0.36), but there was an interaction between DDGS level and phytase (P = 0.03). Pigs fed CS+PHY (55.0%) had higher (P = 0.02) P digestibility than pigs fed CS-PHY (46.6%). Similarly, feeding DDGS+PHY (46.6%) led to improved (P = 0.02) P digestibility compared to feeding DDGS-PHY (42.3%). The addition of phytase to the diet reduced (P = 0.008) fecal P excretion (CS-PHY, 5.15 g/d; CS+PHY, 3.76 g/d; DDGS-PHY, 5.40 g/d; DDGS+PHY, 4.05 g/d). There were no interactions (P > 0.1) between DDGS and phytase level for DM, N and P digestibility. In conclusion, digestibility of P was improved and excretion of P was reduced in pigs fed corn-soybean meal and DDGS diets containing phytase.
The addition of phytase to the diet reduces the concentration of P in the manure (both feces and urine) while adding 20% DDGS to the diet did not. These results suggest that agronomic application rates of manure from diets with DDGS are expected to be similar to those for manure from the standard corn-soybean diet. Results from the feeding trial of this study documented that diets formulated with either phytase or DDGS reduced the concentration of water soluble P forms in the manure compared to the conventional diet, suggesting that neither diet is expected to increase risk of P movement in runoff or leaching. A detailed incubation study was conducted that combined manure from each of the experimental diets with agricultural soils derived from three different parent materials. The incubations mimic a typical manure application rate with soil incorporation. Water extractable P was always the least for the control soils without P amendments and was greatest for soil amended with fertilizer P. Concentrations of plant available P were greater than concentrations of water soluble P for all treatments and incubation times. Soil type affected the concentrations of both water soluble P and plant available P. Plant availability of P in manure from diets with phytase or DDGS is similar to the availability of P in manure from a standard diet.